Carbon fibre is widely used in high-tech industrial field due to its excellent properties such as low density, high strength, high modulus, high temperature resistance, corrosion resistance, friction resistance, and fatigue resistance, etc., especially has a very potential application in aerospace field. The production of carbon fibre generally comprises spinning, pre-oxidizing and carbonizing process.
The properties of carbon fibre, to a great extent, depend on its precursor fibre. The low quality of polyacrylonitrile (PAN) based precursor fibre has been a “bottleneck” restricting the development of carbon fibre industry in china for many years. It is urgent to effectively improve the quality of PAN precursor fibre, thereby improving the properties of carbon fibre. Compared with precursor fibre produced abroad, homemade precursor fibre has larger fineness, lower strength, larger dispersion coefficient, more defects, cracks and voids, lower crystallinity and orientation, etc., which are serious problems existing during production of precursor fibre. As far as quality and yield of precursor fibre are concerned, quality is the primary problem at present. The tensile strength of most carbon fibres produced from homemade precursor fibre is about 3.5 GPa, which can not fulfill the requirement for use at present, therefore its application is limited. Meanwhile, the poor stability of precursor fibre quality is an obstacle to scale production.
A main feature of PAN resin is its high melting point (317° C.). It is decomposed before melted when it is heated, therefore only solution spinning can be used to produce PAN fibre. A large amount of toxic or corrosive chemical solvents are required in industrialized wet spinning and dry spinning, and recovery and purification of the used solvents, washing fibres with water and drying, as well as “three wastes” treatment are necessary during production. If the melt spinning of PAN fibre can be realized, not only solvent exhaustion but also recovery step and devices for solvent recovery and washing step can be saved, therefore the manufacturing cost can be substantially lowered, and the serious environmental problems caused by solvents are eliminated.
It was firstly reported by Coxe in 1952 that adding a small amount of water into PAN copolymers can lower its melting point to that required for melt spinning This report provided a possibility for melt spinning of PAN fibre. Since then, especially, the last 20 years, a lot of researches on melt spinning of PAN are carried on abroad by many foreign companies such as ACC Co., Du Pont Co., BP Chemical Co., Mitsubishi Rayon Co., Ltd., Exlan Co. Ltd, Asahi Kasei Corporation, etc.
In general, there are two ways for melt spinning of PAN: plasticized melt spinning and non-plasticized melt spinning, wherein plasticized melt spinning comprising the following aspects: {circle around (1)} Plasticized by solvent (such as DMSO and PC and the like): PAN powder which had been plasticized by PC can be melted and be extruded continuously into filaments. For example, the study on Rheological properties of mixture of PAN and PC in weight ratio of 50:50 at 180° C. and 240° C. shows that the blend fluid thereof is shear thinning fluid and its viscosity is lower than that of conventional plastic PE; {circle around (2)} Plasticized by non-similar polymer such as PEG reported in literatures: PAN fibre is prepared by melt spinning of PAN and PEG mixture by Asahi Chemical Co. Ltd, the tensile strength of which can be up to 4.68 cN/dtex; {circle around (3)} Plasticized by low molecular weight PAN: 91 parts copolymer of PAN and methyl acrylate (copolymerization ratio being 85:15 by weight, specific viscosities being 0.68) and 9 parts another copolymer of PAN and methyl acrylate (copolymerization ratio being 85:15 by weight, molecular weight being 4800) are mixed and melt extruded at 215° C., and spun at 1200 m/min to obtain fibre, which is drawn in boiling water to 4 times to obtain fibre having a linear density of 1.17 dtex, a tensile strength of 5.26 cN /dtex, and elongation at break of 12.3%, as reported by Mitsubishi Rayon Co., Ltd. And fibre satisfying certain requirements can also be melt spun by reducing AN unit content of low molecular weight PAN for plasticizing appropriately; {circle around (4)} Plasticized by water, which is the most studied method: PAN and certain amount of water become melt under a certain pressure and temperature, which is then extruded into spinning pack and then spinning duct through spinning machine, and drawn. There is full of water vapour in the spinning duct to prevent fibre foaming due to rapidly water evaporation. The obvious characteristics of this method lie in that the only use of inexpensive and non-toxic water will save recovery procedures and devices and will not produce pollution to the environment. It was reported in literatures that PAN fibre obtained from melt spinning by using water as plasticizer can be used as precursor fibre for carbon fibre and have a molecular weight of 100,000-250,000, strength of 3.6 cN/dtex, Young modulus of 97 cN/dtex, and the carbon fibre obtained by carbonization has an average strength of 15 cN/dtex, Young modulus of 1080˜1310 cN/dtex and sonic modulus over 1000 cN/dtex. Recently, aerospace grade carbon fibre prepared from PAN fibre as precursor fibre obtained from melting spinning and plasticized by water is also developed by Celion Carbon Fibres Company. However, this method also has the following problems: A. The extrusion pressure of screw is relatively high due to the poor rheological properties of hydrous melts; B. To prevent the surface of fibre from being coarse and microvoids being formed thereon which result in poor mechanical properties of fibre due to too quick water evaporation during coagulation, saturated steam of certain pressure is required to be maintained in the spinning duct, thereby presenting a requirement for devices; C. It is difficult to control the process due to the narrow temperature range for melt spinning of hydrous melt, therefore industrialization of melt spinning of hydrous melt has not been realized yet at present.
During the manufacturing of carbon fibre, pre-oxidization is a key procedure and the most time-consuming process, the structure and properties of the final carbon fibre to a great extent depend on the structural transformation during pre-oxidization. Since there is dramatic structural transformation during pre-oxidization, defects are easily caused, resulting in a decrease of the mechanical properties of carbon fibre. Therefore the structure transformation and control of structure during pre-oxidization are very important to the control of the structure and properties of carbon fibre.
All processes for pre-oxidizing PAN precursor fibre for carbon fibre reported in literature till now pre-oxidize precursor fibre in air. That is to say, all PAN based carbon fibres are obtained by spinning, pre-oxidizing and carbonizing processes sequentially. However, such an order will result in the following shortcomings: 1) During pre-oxidization of PAN precursor fibre, uneven morphological structure of fibre will be caused if there is a gradient difference of pre-oxidization degree across the cross-section of fibre, such as common skin-core structure, which will result in uneven radical contraction across the cross-section of fibre and poor preferred orientation and tensile performance, and therefore decrease of properties of final carbon fibre. 2) The pre-oxidization process is very time-consuming, its temperature is high and equipments are complicated, resulting in the improved cost for pre-oxidization and therefore eventually, the whole cost of manufacturing carbon fibre are substantially increased. Therefore, the pre-oxidization of PAN fibre is very important. Pre-oxidization is a systemic engineering, revolves in not only equipments and means of pre-oxidization, but also process parameters (such as temperature, time, drawing, medium, flow and direction of medium), reaction and change in PAN fibre during pre-oxidization, evaluation index of structure and pre-oxidization degree of pre-oxidized fibre, etc.
In recent years, researchers at home and abroad take more and more efforts on pre-oxidization of PAN precursor fibre. However, their researches are carried on pre-oxidization of PAN precursor fibre after spinning For example, PAN precursor fibre are also pre-oxidized by three huge companies which produce carbon fibre, namely, Toray, TOHO, Mitsubishi Rayon. Pre-oxidization is double diffusion process and oxygen diffuses from surface to inside of fibre. As the pre-oxidization reaction continues, compact thin layer with ladderlike structure is formed on the surface of fibre at first, blocking the diffusion of oxygen, and then a skin-core structure is formed, resulting in defects of carbon fibre.
Manufacturing process using layer-based thermal stabilized furnace with 6-12 zones for heating and drawing which can produce pre-oxidization fibre with high quality is disclosed in Chinese Patent No.s 02136722.1 and 200810036189.4. However, the equipments for this process are extremely complicated, the temperature is difficult to control and the cost is high.
The industrial objects of carbon fibre production are to lower the cost, improve the properties and productivity of carbon fibre. For quick pre-oxidization and excellent pre-oxidized fibre, the process of pre-oxidization should be optimized. The key for lowering production cost is shorten the time for pre-oxidization which causes easily skin-core structure and subsequently larger voids and defects during carbonizing procedure, thereby resulting in decrease of mechanical properties of carbon fibre Skin-core structure is not obvious by lowering temperature and prolonging time of pre-oxidization, which is beneficial to properties of carbon fibre, however, also lowers the production efficiency. Thus an excellent process for pre-oxidization has not been developed yet.
During the production of carbon fibre (or graphite fibre), especially carbon fibre obtained by using PAN precursor fibre as starting material, the formation of voids on surface is caused by defects of precursor fibre itself and evenness problems during production. Those voids induce stress concentration when fibre is subjected to force, which is also the main reason for break of monofilament. Repairing voids on surface has been concerned in carbon fibre production field, but there is not good means for it until now, and the only way at present is to sacrifice monofilaments with voids, therefore the overall mechanical properties of carbon fibre are substantially decreased.
As disclosed in Chinese Patent No. 02121070.5, environment of an ethyne reaction is created by heating through focusing electromagnetic field induction to induce ethyne to be cracked into hydrogen and carbon atoms near high temperature carbon fibres. Carbon atoms deposits on the surface of carbon fibres to repair defects on surface thereby reinforcing carbon fibre. However, the equipments for this process are very complicated and costly, uneasily to handle and the efficiency is low.